Kleefie just posted an excellent series of Blogcasts that run through the basics of configuring Quarantine for VPN solutions.
He's done the demo using the original Resource Kit Quarantine server (aka Agent) and listener sample (RQS and RQC respectively), but Windows Server 2003 SP1 actually includes a PSS-supported version of each.
If you're not on Windows Server 2003 SP1 - which includes the RQS Agent in Add/Remove Programs - there's a more recent version of the tools on the Download Center here than was featured in the Reskit (install the Resource Kit Tools version first), and an ISA 2004 set of additional utilities here.
Are you sitting comfortably? Good. Ethel the Aardvark was going...